The invention relates to devices for orienting payloads and is of general applicability to mechanical devices, however, the tracking of moving objects is of particular interest in industrial and scientific applications. In such applications, highly accurate and rapid motion is essential. For example, tracking of flying objects for military and civilian use, robotic manipulation and vision, optical inspection and camera applications, machining and polishing operations, etc. can benefit from extremely precise and rapid orienting movements.
The invention is particularly directed to spherical orienting devices which in general provide a large work space for the oriented payload, flexibility and simple geometry.
Prior art for the spherical orienting devices suffer from several disadvantages. Often the mechanisms are extremely complex or require precision machining for accuracy. For example, in U.S. Pat. No. 4,628,765 to Dien et al., a spherical robotic wrist joint is described wherein an oriented object is pointed along an orientation axis passing through the spherical center of rotation. Dien et al. utilize two rotary actuators to drive semi-circular slotted arc members that in combination can orient a radial member within a substantially semi-spherical work space area. The advantage of utilizing only two rotary actuators is substantial, however, the advantage gained is quickly lost in the need for precision machining of the slide faces. Sliding guides are indirectly driven by the actuators inducing friction and the possibility of inaccuracy due to machining tolerances and wear over time.
Another example is provided in U.S. Pat. No. 4,878,393 to Duta, which provides a series of semi-circular hoops interacting with a sliding guide to spherically orient a radially extending arm. In this example as well, the introduction of sliding components reduces accuracy and requires high precision machining. Friction additionally introduces resistance to rapid motion and the possibility of heat buildup.
An improvement is introduced in U.S. Pat. No. 5,243,873 to Demers, providing a two-axis motion mechanism which includes roller bearings thereby improving accuracy and reducing the machining demands. However, Demers retains a single sliding guide and so does not completely eliminate the problems with the prior art.
In the above described prior art, the reliance on sliding guides compromises accuracy and induces friction. The inclusion of bearings and high precision machining does not completely eliminate the disadvantages of the system. Sliding guides are unreliable, particularly in applications where high angular velocities and accelerations are produced and extremely stable mechanisms are essential.
A three degree-of-freedom orienting device utilizing parallel spherical mechanisms has been described in the prior art (article entitled "The agile eye: a high performance three degree-of-freedom camera-orienting device", Clement M. Gosselin and Jean-Francois Hamel 1050-4729/94 IEEE, p. 781). In this article, a prototype of camera-orienting device is described. The device is stable and capable of very large angular velocities and accelerations required for tracking applications, however, due to the three degrees-of-freedom, the mechanism is very complex and contains several moving parts, all of which must be extremely accurately machined. It has been found by experience, however, that the third degree-of-freedom, i.e. torsion of the oriented camera lens, is not required for many applications. In particular, for camera orienting the maintenance of horizon is desirable as a visual reference.
In academic literature, the optimization of a spherical five-bar mechanism driven by two rotary actuators fixed to a base has been analyzed (article entitled "Optimization of Spherical Five-bar Parallel Drive Linkage", Mohamed Ouerfelli and Vijay Kumar, DE-Vol 32.-1, Advances in Design Automation 1991- Vol. 1, ASME 1991).
In this analysis a five-bar spherical mechanism is described wherein two rotary actuators are fixed to a base and orient a payload with axis radiating from the spherical center of rotation. Each joint in the mechanism has an axis of revolution passing through the spherical center of rotation and each link between joints subtends an angle of 90.sub.--. This geometry somewhat simplifies the analysis and results in a spherical linkage of components which are relatively simple to manufacture.
However, in the Ouerfelli and Kumar article, the oriented axis of the payload is disposed on an axis of rotation of one of the joints. This disposition eliminates the possibility of positioning the payload at the center of rotation. As a result, the potential for mechanically interference with links and joints increases since the payload is positioned at a distance from the spherical center of gravity. In addition, positioning the payload away from the spherical center of gravity increases inertia of the system and increases instability of payload support. In any case, the analysis is of a theoretical nature and d does not suggest the structure of a working prototype.
It is an object of the present invention to provide a simple rugged two degrees-of-freedom spherical orienting device capable of accurate orientation with large angular velocities a nd accelerations
A further object of the invention is to provide a spherical orientation mechanism which can support a payload at the center of rotation thereby providing a large workspace, simplified kinematic computations, reduced inertia and reduction of interference between the payload and mechanism within a relatively large internal free operating space.